The formation of single-sided groove structures, as have long been used in Fresnel lenses, makes it possible to provide lenses which achieve short intercept distances with a planar design and a large relative aperture. In this embodiment, the thickness of the Fresnel lenses is virtually constant across the diameter. The groove structure is formed by a sequence of concentric active edges which effect the imaging. The technology dictates that disturbing edges are present between the individual active edges, which disturbing edges lead to undesirable light losses and a corresponding ring structure becomes discernible in the imaging. This is the case particularly when the light is directed onto the plane surface of the Fresnel lens.
The deflecting effect can then be utilized only until total reflection occurs at the active edges. If the light were deflected to a great extent, further light losses always occur due to surface reflections according to Fresnel""s equations, which also lead to partial polarization of the light.
Fresnel condensers having two Fresnel lenses are used in overhead projectors. In this embodiment, the structured surfaces of the Fresnel lenses are opposite one another, such as as shown in FIG. 1. As a result of the short finite back and front focal distances compared with the free aperture of the Fresnel condenser, the refractive power has to be shared between two lenses. In this embodiment, between the two Fresnel lenses, the light rays run parallel, but at the very least virtually parallel to the optical axis.
For liquid crystal display (LCD) projectors, the solution known for overhead projectors was initially resorted to and, in the simplest case, the LCD panel was placed onto such a projector. For good to optimal projection conditions, a virtually perpendicular incidence of light over the entire area, to be projected, of such an LCD display is necessary for good color rendition and high contrast. For this reason, the LCD was transilluminated virtually perpendicularly using a condenser system. The condenser system comprises a Fresnel lens and a conventional glass condenser lens stranding near the lamp. A second Fresnel lens is arranged as a field lens after the LCD display, which lens images the light source in the entrance pupil of the projection lens. A conventional Fresnel lens was used as the field lens and brings about the light losses already mentioned.
The object of the invention includes an optical arrangement using Fresnel lenses which has reduced light losses and allows a large intercept distance on one side. The optical arrangement includes at least two Fresnel lenses which are each structured on one side and the structured surfaces are arranged such that structured surfaces point toward one another. Although Fresnel lenses are exemplified, other types of optical elements can be used, such as linear prisms. The active edges of the two Fresnel lenses are oriented with respect to one another in such a way that, in the event of parallel light incidence on, or light emergence from, a plane surface of one of the two Fresnel lenses. The refractive powers are divided between two surfaces and light deflection is effected only at two of the four surfaces of the two Fresnel lenses.
According to the invention, an arrangement formed from two lenses each with a Fresnel structure formed on one side is used. The Fresnel structures of the lenses are directed toward one another. In this embodiment, one of the two Fresnel lenses is designed and oriented with respect to the other lens in such a way that the light entering or emerging from its active edges enters or emerges at an angle which is at least virtually a right angle.
In order to further reduce the light losses, the disturbing edges of this Fresnel lens should form an angle of approximately ninety degrees with the associated active edges. It is important that only two of the four surfaces through which the light enters or emerges make a significant contribution to the deflection of the light. The light rays do not run parallel to the optical axis.
As a result, it is possible, surprisingly, not only to increase the luminous efficiency but also to suppress the imaging of the Fresnel structure. Moreover, the scattered light component and reflection losses can be reduced. Depending on the side of the optical arrangement according to the invention at which light enters, a virtually infinite front or back focal distance can be achieved. If such an arrangement is used in accordance with the conventional Fresnel condenser in an overhead projector, a virtually infinite front focal distance is achieved if virtually parallel light is radiated in.
The spacing of the Fresnel structures of the arrangement according to the invention should be as close as possible in order to achieve a short structural length and, if appropriate, to connect the Fresnel lenses to one another in order, in this manner, to protect the Fresnel structures against dust and mechanical damage. The alignment of the two Fresnel lenses with respect to one another is not as critical as in the case of a conventional overhead projector lens, because the light is virtually perpendicularly incident on the active edges of the second Fresnel lens.
If the invention is used for overhead projectors, it is expedient to design the two Fresnel lenses to be slightly bent or to hold or arrange said lenses in a slightly bent position in order to improve the uniformity of the image field illumination and increase the mechanical stability. Moreover, it is possible to reduce light losses due to surface reflection by virtue of the smaller angles of incidence at the surfaces. This can further be improved by an antireflective surface or coating, where conventional coatings or layer systems can be used for this purpose or other reflection-reducing methods can be employed, such as subwavelength structures (for example, moth-eye structures), for this purpose.
In certain embodiments it may be expedient to produce the two Fresnel lenses from different materials, that is to say with different refractive indices. In this embodiment, the deflection of the light may be influenced in addition to the angles at which the active and disturbing edges of one Fresnel lens are formed. The active and disturbing edges of the Fresnel lens which deflects the light at the active edges are meant in this embodiment.
By dividing the refractive powers between two surfaces, it is possible to avoid the limitation of the maximum deflection by total reflection, as occurs according to the prior art, if the light from the light source impinges on a plane surface of one of the two Fresnel lenses.
The arrangement according to the invention can be used equally as a converging lens or diverging lens. Moreover, a combined use with a further optical element which can be arranged in the beam path of the light is readily possible and additional diverse applications are opened up in accordance with the respective optical element. Such optical elements may be, e.g., prisms, such as linear prisms, beam splitters, cylindrical lenses or lenticular lenses. An additional Fresnel lens may expediently be used as such an optical element, which lense may be arranged parallel to the two Fresnel lenses.
The invention may be used particularly advantageously in overhead projectors, in LCD projectors or other displays, such as e.g. back projectors, video projectors (cathode ray tube (CRT) or LCD), laser display projectors, but also in array arrangements of such projection systems (Video-Walls), the known disadvantages being avoided to the greatest possible extent.